Navigation systems are increasingly dependent upon the global positioning system (GPS). The navigation receiver should be able to withstand intentional or unintentional signal interference with robust signal acquisition and tracking. For this reason, inertial navigation systems have been coupled to GPS signal tracking and acquisition systems for improved signal tracking and acquisition of the received signal. Inertial navigation systems (INS) have an inertial measurement unit (IMU) for processing inertial measurements. Coupling navigation data with the GPS signal tracking and acquisition system improves signal tracking and acquisition. When the inertial measurements of a navigation processor are used with a GPS signal tracking and acquisition system, the combined system is tightly coupled. The tightly coupled GPS and INS system uses validated pseudorange and pseudorange rate measurements from the GPS receiver instead of the processed position and velocity measurements. In addition, inertial measurement unit data is usually used to assist the receiver tracking loops and enable more noise filtering than would be possible without tracking loop aiding. The tightly coupled method has been used with the validated pseudorange and pseudorange rate measurements. Ultratight coupling systems have been disclosed in U.S. Pat. No. 6,516,021. More recently, ultratight GPS and INS coupling has been proposed which drives all signals from the INS navigation solution for a period of time then examines all signals with respect to the error of the navigation solution. Ultratight receivers do not attempt to control code replicas for exact code phase alignment with the incoming signal but merely seek to observe the deviation of the incoming signal from locally generated replicas and feed that deviation information back to the code replica generators. Ultratight coupling has shown to be more robust in the presence of noise. Ultratight coupling does not require that the replica code track or lock onto the incoming signal. Uncoupled systems, loosely coupled systems, and tightly coupled systems, have conventional lock detectors that are known to validate measurements for improved performance. The output lock detectors measure the performance of the system. However, new ultratight validations and performance metrics have been sought but no proposed metric has been accepted.
A tracking loop is a combination of electronic hardware and software algorithms used to track a pseudorandom noise (PRN) code signal and the signal carrier. A tracking loop that tracks the PRN code is a code-tracking loop. A tracking loop that tracks the carrier is a carrier-tracking loop. The carrier-tracking loop can track the phase or the frequency of the carrier or a combination of both. A carrier-tracking loop design is adjusted to each application by designing the closed loop gains and the order of the filter to obtain the desired filtering and dynamic response. The code-tracking loop attempts to drive the code phase of a replica signal to be aligned with the received signal, so as to enable coherent demodulation of the received signal. The tracking loops include a signal generator that generates an estimated replica signal of the received signal using a control signal that advances or retards in time the code replica signal relative to the received signal. The code-tracking loop includes a correlator that multiplies the received signal by the code replica signal and passes the multiplied signal result through a low pass filter. The code-tracking loop includes a discriminator generator that generates a discriminator signal having a value related to the difference between the received signal and the replica signal. The code-tracking loop also includes a controller that filters the discriminating signal into the control signal that is then communicated to the signal generator.
A fundamental function in all GPS receivers built to date is the ability of the receiver to generate a code replica signal of the received signal transmitted by a satellite that can be correlated with the code of the received signal being received from the satellite. The code replica signal is advanced or retarded until the locally generated replica signal correlates with the received code in a receiver. For loosely or tightly coupled GPS and INS systems, the code-tracking loop is performed by code and carrier-tracking loops that determine an error signal that is a measure of the range and range rate difference between the generated and received signals. That difference between the received and replica signal is an error signal that is processed by a transfer function for generating input values communicated to a numerically controlled oscillator to advance or retard the generated code replica signal. When the code replica signal correlates with the received code, the tracking loop is in lock and the feedback error signal is near zero. When the code and carrier tracking loops are in lock, the state of the code and carrier generation process can be sampled to obtain a measure of the pseudorange and pseudorange rate. The pseudorange is the geometric range between the transmit antenna and receive antenna plus a bias due to the user clock error. The pseudorange rate is approximated by the amount of range change for a predetermined amount of time plus a bias due to the drift of the local oscillator frequency.
The pseudorange and pseudorange rate measurements are predicated upon the assumption that the replica is aligned with the incoming signal. Lock detectors are used to determine when the carrier and code replicas are coherently aligned with the incoming signal in terms of code phase by a code lock detector, of the carrier frequency by a frequency lock detector, and of the carrier phase by a phase lock detector. When the code lock detector indicates that the code of the replica is in lock with the received signal, then the pseudorange measurement, derived from an instantaneous sampling of the state of the code generator at a desired measurement epoch time, is considered valid. When the frequency lock detector indicates that the carrier frequency is in lock with the carrier phase of the received signal, then the pseudorange rate measurement is considered valid. The pseudorange rate measurement is obtained by strobing the carrier replica twice over a small period of time and determining the change in carrier phase or delta pseudorange over a discrete period of time. In the limit, as the time interval goes to zero, the ratio of the delta pseudorange divided by delta time approaches the instantaneous time rate of change of the pseudorange, which rate of change is the pseudorange rate.
Lock detectors improve navigation performance for loosely and tightly coupled systems, because the lock detectors prevent bad measurements from being sent to the integration Kalman filter. In addition to improving navigation performance, the signal to noise levels at which a receiver loses code lock, carrier frequency lock, and carrier phase lock are used as measures of receiver performance.
The code-tracking loops that track the incoming satellite received signal must adjust the phase and the frequency of the generated replica signal for many changing variables, such as user and satellite relative motion and user clock drifts. These tracking loops are traditionally called the code loop and the carrier loop, because the code loop tracks the phase of the pseudorandom noise (PRN) code and the carrier loop tracks the signal carrier frequency and the carrier phase. Although a phase locked loop is the most common way to track a carrier signal, the GPS signals have a fifty hertz navigation data message superimposed on the code generation process, which can potentially change the phase of the signal by 180° every twenty milliseconds. To avoid a loss of lock during this change of phase, a Costas tracking loop is used in place of the conventional phase lock loop. This allows the carrier to be tracked across 50 Hz data bit changes with no loss of lock. Frequency lock loops are also used for the carrier tracking and sometimes are used in combination with phase lock loops to improve robustness. When the tracking loop error builds beyond a certain threshold value, the loop is deemed to be out of lock and pseudorange and pseudorange rate measurements are not used until the loop reacquires lock.
Ultratight coupling generates all replicas based on a single navigation estimate for a period of time called an epoch. The replicas are not required to precisely track the incoming signal during the epoch but must remain within about a chip to ensure the signal is in view. The correlation outputs for every signal from a satellite are modeled by a function of the error between the navigation state estimate and the true navigation state, total electron content error ΔTEC, and error rate ΔTEC′. These errors are then estimated based on examination of the correlation outputs. Ultratight coupling has been shown to be a more effective way of processing GPS and IMU signals in the presence of noise, and has resulted in improved estimates of the ionospheric effects. Ultratight systems can use information from other satellites to propagate a replica of a satellite that might momentarily be obscured, and hence can quickly reacquire a signal without going through a lengthy acquisition process. There are problems during detection when a particular satellite is giving poor information or when identifying when an ultratight system is not receiving valid information from GPS signal processing. Because ultratight receivers do not lock onto the incoming signal, lock detectors cannot be applied to the correlation outputs to determine the validity of estimates. The validity of the estimates is not determined. As such, invalid estimates can be passed to the integration Kalman filter as measurements. The integration Kalman filter will have no way of determining when the measurements are invalid. The integration Kalman will apply the invalid measurements resulting in a degraded navigation estimate. The degraded navigation estimate can cause one or more replicas to be out of the field of view of the signal resulting in greatly reduced navigation performance. Because ultratight systems never lock onto the signal, the signal to noise ratio at which the receiver falls out of lock cannot be applied as a performance metric. Ultratight navigation systems disadvantageously lack an ability to quantify performance and determine when incoming signal observations are valid. These and other disadvantages are solved or reduced by the present invention.